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12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 251

modified plants compared to wild-type (WT) controls (Arazi et al. 1999; Sunkar et al.
2000). Intriguingly, the same transgene conferred improved Ni tolerance, which was
apparently due to NtCBP4-promoted Ni-exclusion by yet unidentified mechanism.
Expression of the Zn-uptake transporter NcTZN1 gene (ZIP family) from the asco-
mycete Neurospora crassa in N. tabacum substantially promoted accumulation of
Zn, but not Cd (Dixit et al. 2010). When grown hydroponically in media amended
with 70 μM Zn, the Zn concentrations in roots and shoots of transgenic tobacco were
6.5- and 2-fold higher than those in WT plants. Under Cd stress (200 μM Cd),
N. tabacum expressing MTF-like transporter genes BjCET3 and BjCET4 from
B. juncea showed higher Cd tolerance and tobacco producing CET4 accumulated
twofold more Cd in shoots than control plant, while maintaining similar shoot
biomass production with controls (Lang et al. 2011). The overexpression of hypo-
thetical plant iron transporters of the NRAMP family in model plants A. thaliana or
N. tabacum has been primarily conducted to assess their function in Fe homeostasis
(Curie et al. 2000). It was also found that the overproduction of intrinsic AtNRAMP3
in A. thaliana markedly increased sensitivity of the transgene to Cd, consistent with
an idea of remobilization of Cd from vacuole (Thomine et al. 2000). However, this
phenotype was not accompanied by increase in the net Cd accumulation. Promoted
metallotolerance and translocation of Cd and Pb was achieved in B. juncea upon
expression of mitochondrial Cd-efflux ABC transporter AtATM3 from A. thaliana
(Bhuiyan et al. 2011a). The best performing transgenic lines contained in their shoots
up to 2.5-fold higher levels of both metal ions than WT. It should be noted that
AtATM3 is not expected to localize in plasma membrane in transgenic plant; still it
can be intricately involved in translocation. The authors attributed enhanced trans-
location to increased Cd and Pb levels in the cytoplasm which stimulated PC and
GSH synthesis as well as expression of several intrinsic metal transporters.
The feasibility of using bacterial metal transporters in plants was first
demonstrated in A. thaliana transformed with zntA coding for Zn, Cd, and Pb P1-
ATPase responsible for the metal-efflux-based metalloresistance of Eschericha coli
(Lee et al. 2003). In transformed A. thaliana, localized ZntA on plasma membrane
reduced the Cd accumulation in protoplasts by promoting release of preloaded Cd.
Overall ZntA improved the tolerance of the ZntA plants and shoots of transgenic
grown at these concentrations showed, respectively, decreased content of Cd and
Pb, a desirable feature for crop plants to be safer from heavy metal contamination.
In N. tabacum, the expression of a bacterial proton-motive force drive transporter of
the NiCoT family from Rhodopseudomonas palustris increased the accumulation of
Co in shoots of hydroponically (42 μM Co in media) grown twofold (Nair et al.
2012). On the whole plant level and compared to the control, transgenic plant
accumulated up to 5 and 2 times higher concentrations of Co and Ni, respectively,
while uptake of Fe, Cd, Zn, and Cu remained unaffected.
The widespread bacterial Hg resistance mechanism, based on the import of Hg
into cytoplasm and its subsequent reduction to metallic mercury involves MerT and
MerC as one of the plasma membrane transporters for the Hg uptake step (Silver and
Phung 2005). In a model experiment with merC-expressing A. thaliana, the leaves
when excised and submerged into a solution containing 100 μM Hg, showed more